The invention relates to an arrangement for facilitating the testing of a device comprising a transmitting circuit and a receiving circuit coupled with a transmission line by means of a hybrid junction, the device comprising an echo canceller provided with a difference circuit which has one input connected to the receiving end of the hybrid junction, the other input to an echo copy signal generator and, finally, the output to the said receiving circuit, said test consisting of comparing the signal restituted by the receiving circuit and the signal applied to the transmitting circuit when the device to be tested is fed back at the transmission line side.
The device is to be tested may be, for example, a data transmission modem whose emitting circuit is an emitter modulating a carrier as a function of the data to be transmitted over the transmission line and whose receiving circuit is a receiver restituting the data received from the transmission line. The echo canceller incorporated in this modem has for its object to prevent any signal coming from the transmitter and appearing at the receiving end of the hybrid junction (owing to faulty operation of this junction or to echoes on the transmission line) from reaching the input of the receiver. Such a signal, which is denoted echo signal is cancelled at the output of the difference circuit of the echo canceller.
For testing a modem it is customary to effect the feedback from the transmission line side, the feedback consisting in that in one way or another a feedback signal, which corresponds to the signal produced by the transmitter, is made to appear at the receiving and of the hybrid junction. Testing a modem which is fedback in this manner and to whose transmitter a test sequence is applied must consist of a verification whether the signal restituted by the receiver is identical or not identical to this test sequence. When such a test system is used for a modem comprising an echo canceller, the following problem is met. The feedback signal appearing at the receiving end of the hybrid junction during the transmission of the test sequence is processed by the echo canceller as an echo signal and is consequently cancelled at the output of a difference circuit. Therefore, during the test, a proper operation of the echo canceller results, in practice, in an absence of the signal at the input of the receiver, having for its result that the signal restituted by the receiver is indeterminate and, at any rate, different from the test sequence applied to the transmitter. A test which is thus effected does not furnish any indication on the fact whether the modem functions properly or poorly. If then a further test operation is performed in which the echo canceller is disconnected, it is possible to get information on the quality of operation of the transmitter and receiver of the modem. But even after two tests of the modem, with and without echo canceller, there is not any indication on the operation of the echo canceller in the modem.